Support eth_chainId RPC method

[hackmod]

• EIP: Implement eth_chainId method in JSON-RPC ethereum/EIPs#695
• Original PR is Add RPC eth_chainId for querying the current blockchain chain ID #6329 by @sorpaas

This PR copy the original PR #6329 work to support both eth_chainId and parity_chainId temporary

parity_chainId could be dropped later.

• EIP:695 - Implement eth_chainId method in JSON-RPC - EIP: Implement eth_chainId method in JSON-RPC ethereum/EIPs#695 accepted(Aug 22 2017)
• eth_chainId PR for Ethereum (geth) - eth,internal: add eth_chainId method to PublicEthereumApi JSONRPC ethereum/go-ethereum#15002, add eth_chainId method ethereum/go-ethereum#17617 merged(9/9)
• eth_chainId PR for ETC (classic-geth) - implement 'eth_chainId' JSON-RPC method ethereumproject/go-ethereum#336 merged(Aug 16 2017)
• parity_chainId PR - Add RPC eth_chainId for querying the current blockchain chain ID #6329 merged (Aug 18 2017)

See also MetaMask/metamask-extension#5552

(Edit: for release note, we need to mention that this replaces parity_chainId with eth_chainId.)

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[hackmod]

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[seunlanlege]

LGTM

[sorpaas]

Please also drop parity_chainId in this PR. The original intention to use partiy_ namespace rather than eth_ namespace is because the EIP was not standardized. That concern is apparently already addressed as of now.